1. Field of the Invention
This invention relates to visual, irreversible time-temperature history indicators useful for indicating whether perishables, such as foodstuffs, pharmaceuticals, chemicals, photographic films and the like, have experienced undesirable time-temperature histories which result in substantial degradation or whether products have been exposed to desirable time-temperature histories during processing.
2. Description of the Prior Art
The need to know whether a product has been exposed either to an undesirable time-temperature history which results in substantial degradation or to a correct time-temperature history required during processing is self-evident. This applies, for example, to frozen foods, pharmaceuticals or photographic films which may be exposed to undesirable high temperatures for significant time periods during storage and distribution. This also applies to canned goods and biomedical materials, which must be held at high temperatures for a specific time period in an autoclave to guarantee sterilization.
A number of patents have issued disclosing indicators useful, for example, in detecting whether frozen foods have been exposed either to time-temperature combinations or to a particular temperature which results in substantial degradation. U.S. Pat. No. 2,553,369, issued May 15, 1951 to S. Hoffman, discloses a time-temperature indicator comprising a combination of hydrated potassium triiodide, a soluble starch and a diastase. At temperatures above freezing, the diastase (which is a starch-digesting enzyme) hydrolyzes the starch, thereby altering the blue color initially rendered to the starch by the potassium triiodide.
U.S. Pat. No. 2,892,798, issued June 30, 1959 to D. L. Dobbs et al., discloses an irreversible temperature indicator for frozen foods comprising an aqueous solution of mercuric and cuprous iodide colloidally dispersed in a liquid phase comprising an alkali metal iodide. Upon exposure to thawing temperatures, a color change occurs.
U.S. Pat. No. 3,545,400, issued Dec. 8, 1970 to V. L. Smith, discloses a freeze and thaw indicator which is activated by rupture of a dye-filled container upon freezing. Upon thawing, the released dye flows onto an absorbent pad, thereby providing the positive indicator response.
U.S. Pat. No. 3,768,976, issued Oct. 30, 1973 to K. H. Hu et al., discloses a process for constructing a time-temperature history indicator. The operation of this indicator depends upon the rate of permeation of oxygen through a polymer envelope containing an aqueous solution of a redox red dye. Upon oxidation, the red dye turns colorless, providing the warning signal that the perishable has been exposed to too high a temperature for too long a time period.
U.S. Pat. No. 3,844,718, issued Oct. 29, 1974 to H. Cohen, discloses a defrost indicator which is activated by the contact of water or water vapor with a water-soluble ink supported on a hygroscopic substrate.
Indicators have also been disclosed for high temperature applications; see, for example, U.S. Pat. No. 1,668,767, issued May 8, 1928 to J. Hansen et al., which discloses a process for indicating whether cans filled with food have been exposed to sufficiently high cooking temperatures. The process comprises marking the cans with a colored substance which changes color at 212.degree. F. The coloring substance is an organic color such as erythrocine in a binder, such as shellac, and a solvent, such as grain alcohol.
U.S. Pat. No. 3,078,182, issued Feb. 19, 1963 to J. W. Crone, Jr., et al., discloses a color-changing pressuresensitive adhesive indicator tape for indicating whether sterilization of packages has occurred. The indicator comprises a mixture of a halogen-containing binder resin and a heat sensitive heteropolymolybdate pigment.
The foregoing references are representative of many patents in this area. A drawback of many of the indicators in the prior art is, however, that they are useful only over very limited temperature ranges, such as within a few degrees of the freezing point of water, or that they are bulky or expensive, or that they depend on diffusion or on complex reaction mechanisms for their operation. Furthermore, most of these indicators do not provide a direct measure of time-temperature history. This is most important, since both proper product processing and degradation of perishables often depend on the time exposure to particular temperatures. For example, food exposed for a period of time at one temperature may degrade to the same extent as if exposed for a shorter period of time at a higher temperature. Similarly, high temperature processing for a short period of time may achieve the same effect as lower temperature processing for a longer period of time. Thus, the time-temperature history to which an article has been exposed is often more critical than whether it has been exposed to a particular undesirable degradation temperature or to a desirable processing temperature. Indicator materials are consequently required which match the time-temperature degradation characteristics of a wide range of perishables and the time-temperature characteristics of a wide range of production processes.
Accordingly, it is often desirable that a time-temperature indicator should undergo a series of readily detectable changes corresponding to the progressive development of time-temperature history. Thus, the indicator should preferably be used to denote the integral, or sum total, of thermal exposure (time and temperature), rather than merely that a particular temperature has been exceeded. Many of the indicators disclosed in the prior art are not capable of denoting integral time-temperature behavior.
Finally, many of the indicators disclosed in the prior art are not conveniently activated or deactivated. If the indicator cannot be conveniently activated, preusage storage of the active indicator can become a problem. Only if time-temperature history development can be conveniently halted, is it possible to obtain permanent indicator responses at various points in the storage and distribution of a perishable or the processing of a product.
There are many materials that evidence an irreversible color change upon exposure to temperature. For example, polyacetylenes having at least two conjugated acetylene groups have been disclosed in U.S. Pat. No. 3,822,134, issued July 2, 1974 to A. A. Rasch et al., for use as vacuum-deposited radiation-sensitive elements. Some of these radiation sensitive elements evidence an irreversible color change upon exposure to temperature. However, such a color change, by itself, is insufficient to suggest its use as a practical time-temperature history indicator, since the color change may simply indicate that a particular temperature has been exceeded, without an indication either of the length of time that temperature has been exceeded or of the time averaged exposure at higher temperatures. Further, in order to construct a readily readable indicator, a sharp color change which occurs in a narrow time interval for each of a range of exposure temperatures is required.
Practical indicators accumulate a time-temperature history in integrated form as a single reading. Desirably, such indicators should parallel the reaction of the particular perishable product to which they are attached to changes in temperatures over periods of time.